Lignin from white-rotted European beech deadwood and soil functions

Abstract

In forest ecosystems, deadwood can improve carbon storage, nutrient availability, and water holding capacity in soils. Yet the effect of organic matter from deadwood such as lignin on these soil functions and their regulators are unknown. We hypothesized that carbon storage, exchangeable cations, and pore space increase with the quantity of lignin-derived phenolic acids from beech deadwood. We also hypothesize that the most pronounced differences occur in more advanced decay classes, in the forest floor at sites with moder forest floors, and in the Ah horizon at sites with mull forest floors. Cupric oxide-oxidation products were used to determine lignin concentration, composition, and oxidation from paired reference and test samples next to 42 downed European beech (Fagus sylvatica L.) deadwood logs in ten stands in Southwest Germany. Compared to reference points, the sum of vanillyl, syringyl and cinnamyl lignin-derived phenols increased next to beech deadwood (within 10–20 cm). The composition and oxidation of lignin-derived phenols also changed near beech deadwood: syringyl/vanillyl ratios increased while cinnamyl/vanillyl and aldehyde/acid ratios for vanillyl decreased. Water-extractable organic carbon (OC) and its aromaticity also increased next to beech deadwood as did total OC and particulate OC separated by density fractionation relative to total and mineral-bound OC. These changes occurred namely in the organic horizons of moder forest floors, and in the Ah horizon underneath mull forest floors. These observations indicated that phenols predominantly entered soil in fluxes of fragmented and dissolved organic matter from beech deadwood. Changes to soil nutrient availability and porosity were linked to increasing lignin-derived phenols from beech deadwood especially in nutrient-poor soils and near heavily decayed deadwood. This is evidence that soils close to beech deadwood, a substrate, are spatially limited pedogenic hot-spots that have increased soil carbon, available nutrients, and pore space depending on the forest floor and parent material.